1. Field of the Invention
This invention relates to the recovery of fluoride, aluminum and carbon values from waste cathode liner material used to line aluminum electrolytic cells.
2. Prior Art Relating to the Disclosure
Cathode pots of electrolytic furnaces used in the production of aluminum are lined with side carbon and bottom carbon compositions which are electrically conductive. The bottom carbon is generally of graded anthracite coal and coke bonded together with pitch. During electrolytic operation, the cryolite components, rich in sodium and fluoride values, are slowly absorbed into the lining. Eventually the cathode liners foul to the point where they must be replaced. The spent cathode liners contain substantial amounts of fluoride values, aluminum in the form of aluminates, sodium fluoride, and absorbed sodium metal which, on exposure to atmospheric moisture, is converted to a caustic. Other materials, such as the anthracite carbon contained in the monolith spent liner, are valuable. The monolith liner is composed of sidewalls and a bottom wall, with the bottom wall containing the anthracite carbon and constituting about two-thirds of the total weight of the cathode lining. Disposal of the spent potliner has posed a problem due to the leaching of fluorides and other contaminants into ground water.
Methods for recovering cryolite from spent cathode liners are known. Such methods have generally employed (1) caustic (sodium hydroxide), (2) sodium carbonate, or (3) water to extract the fluoride values from used cathode liners. By the first process, crushed carbon cell lining is treated with an aqueous caustic solution to yield water-soluble sodium fluoride and water-soluble sodium aluminate. This solution is processed to precipitate cryolite. Such methods are described in U.S. Pat. Nos. 1,871,723 and 2,732,283.
By the second process, crushed carbon liners are treated at elevated temperature with a water-soluble carbonate to effect reaction between the fluoride values in the spent liner and the added carbonate. The fluorides in the spent liner are converted to water-soluble sodium fluoride and precipitated with carbon dioxide to form a cryolite. Such a method is disclosed in U.S. Pat. No. 3,106,448.
By the third method, water is used to leach the fluoride values from the spent cathode liner.
U.S. application Ser. No. 520,304, filed Nov. 4, 1974, assigned to the assignee of this application and now abandoned, describes a system for extraction and recovery of the aluminum and fluoride values from spent cathode liners by extracting crushed liner material at ambient temperature with a dilute ammonia solution. Sodium fluoride and cryolite are recovered by evaporation of the ammonium hydroxide leach liquor. Alternatively, the sodium fluoride in the leach liquor is precipitated as calcium fluoride. The principal disadvantage of this process is the energy requirement required to concentrate the dilute solution concentrations.
Removal of soluble fluoride from cathode liner minimizes the subsequent leaching of toxic fluorides into ground water in the vicinity of liner disposal sites. In addition, the recovered fluoride is of value and the recovered liner material may be recycled.